I tried using lwjgl with gcj (from cvs) a couple of months ago (end april/beginning of may or something like that) - it almost worked(tm)
Stubs and most of the implementation of java.nio was there in the src - from what I could tell at a casual glance.
Some strange compile errors - guess that gcj "3.4" was quite unstable atm.
So, if I were you I'd have a chat with the gcj developers and see how good their implementation of java.nio is now (it should be better than when I tried...
The only other thing I can think of (right now) that lwjgl uses which gcj might not support is the javax.imageio package (for loading texture images in examples and demos - I guess it's just optional though).